In metal strip lines in which tension is applied to the metal strip and processing is carried out which involves a certain plastic deformation of the metal strip, waviness and strip camber can arise. The waviness is usually found along the longitudinal edges of the strip and the camber can affect the entire width of the strip. The processing under tension with which the invention is concerned can include cold rolling and dressing rolling (skin-pass rolling) of the strip. The dressing rolling can impart the final thickness to the strip and is distinguished from the cold rolling by the fact that it affects only a relatively small reduction of the strip thickness. Dressing or skin-pass rolling is often referred to also as after-rolling or calibration rolling.
Leveling, as this term is used here, usually means the bending of the strip back and forth, commonly by passage through an array of rollers. A typical leveler has four or more rolls and the strip is bent from side to side as it passes into successive contact with the rolls. Since the strip is under tension, such leveling is also referred to as stretch-bend leveling. The invention is also applicable to processing which involves stretching of the strip, i.e. elongation of the strip under the applied tension. The tension can be applied between bridles, the upstream bridle being driven at a peripheral speed which is greater than that of the downstream bridle. The downstream bridle may be braked to generate the tension while the upstream bridle is driven. The term "flattening" as used herein, is intended to refer to reduction in the waviness or camber of the strip or complete elimination thereof. In addition it should be understood that, while the invention is primarily of interest in the processing of strip, it is also applicable to the flattening of sheets and plates of metal.
In practice it has been found heretofore that it is impossible to completely eliminate the waviness or strip camber which develops in metal strip even through the metal strip is subjected under tension to rolling, leveling and/or stretching in the manner described. As a result, an ideal flat strip can be seldom achieved. The term "ideal planarity" means, within the context of the invention, that all longitudinal segments of the strip are of equal length across the width of the strip when the strip is not under load, i.e. the applied tension is removed, and the temperature in the strip is constant. The strip, although a single piece can be viewed as having a multiplicity of adjoining longitudinal zones or segments which collectively make up the full width of the strip and in the case of temperature inhomogeneity may be at different temperatures and may even have different thicknesses and stresses. There are no strict boundaries between such zones and reference to them is not intended to indicate that the zones are either well developed laterally or distinguishable from one another except by the parameter which differentiates them, like temperature.
Generally the nonflatnesses which characterizes metal strip may result in part from residual bending moments which can give rise to curvature in the longitudinal and/or transverse direction, referred to as coilset or crossbow. The invention is not intended to deal with these contributions to the nonplanarity of the strip or with secondary effects which arise because of nonuniform distributions of transverse stress in the strip plane. The invention is intended, however, to deal with the waviness or camber which can result from the effective length differences of longitudinal zones of the strip.
Residual waviness after cold rolling can amount typically to up to 100 I units, after dressing or after-rolling up to 30 I units and after leveling to up to 10 I units. The I unit corresponds to a length difference between two strip zones in the metal strip of 10 .mu.m/m. The planarity can be measured, for example, off line, on a planarity measuring rollers which can signal the effective length differences of the respective longitudinal zones as strip travels past such rollers. In modern rolling technology with control units to improve the planarity for example by roll-bending or axial shifting of the rolling rolls, the planarity of the strip can be greatly improved by comparison to strip produced prior to the advent of such system. Nevertheless an ideal planarity or even an approximately ideal planarity has not, however, been attainable heretofore.
In the rolling of metal strip which has been proposed to improve the planarity by changing the rolling gap geometry over the width of the rolls and to influence the temperature distribution by providing a multiplicity of heating elements in the rolling region, it has also been suggested to supply thermal energy to the metal strip itself so that in thicker longitudinal zones of the metal strip, thermal expansion will be promoted or the elongation of the strip increased over cooler longitudinal zones during the rolling process (see DE 27 43 130). These systems do not however eliminate waviness and strip camber as described.